C.2.20 Timestamp Resolution and Make

Traditionally, file timestamps had 1-second resolution, and
make used those timestamps to determine whether one file was
newer than the other. However, many modern file systems have
timestamps with 1-nanosecond resolution. Some make
implementations look at the entire timestamp; others ignore the
fractional part, which can lead to incorrect results. Normally this
is not a problem, but in some extreme cases you may need to use tricks
like ‘sleep 1’ to work around timestamp truncation bugs.

Commands like ‘cp -p’ and ‘touch -r’ typically do not copy
file timestamps to their full resolutions (see Limitations of Usual
Tools). Hence you should be wary of rules like this:

dest: src
cp -p src dest

as dest often appears to be older than src after the
timestamp is truncated, and this can cause make to do
needless rework the next time it is invoked. To work around this
problem, you can use a timestamp file, e.g.:

dest-stamp: src
cp -p src dest
date >dest-stamp

Apart from timestamp resolution, there are also differences in handling
equal timestamps. HP-UX make updates targets if it has the
same time stamp as one of its prerequisites, in violation of Posix rules.

This can cause spurious rebuilds for repeated runs of make.
This in turn can cause make to fail if it tries to rebuild
generated files in a possibly read-only source tree with tools not
present on the end-user machine. Use GNU make instead.